xenapi: fix where root_gb=0 causes problems

Auto_config disk and resize up should be able to correctly handle when
the flavor specifies root_gb=0.

When you try to resize down to root_gb=0, it doesn't really make sense,
so throw a reasonable error message, instead of failing in the middle of
the resize calculations.

Fixes bug 1152268
Change-Id: Ib26170d7d563cc8fd664c3680fecc4ac04e57b04
Merge "xenapi: fix where root_gb=0 causes problems"